Method for obtaining polymerisable vinyl chloride from a raw product derived from the pyrolysis of 1 2-dichloroethane

ABSTRACT

The invention concerns a method for obtaining directly polymerisable vinyl chloride which consists in subjecting a cooled raw product derived from of pyrolysis of 1,2-dichloroethane, to another cooling process at a temperature not more than 40° C., under pressure ranging between 10 to 15 bars then in leaving it at substantially identical temperature and pressure levels for a duration not more than 20 minutes. The invention also concerns a device for implementing the method.

[0001] The invention relates to a method for obtaining vinyl chloridecapable of polymerization, from a raw product derived from the pyrolysisof 1,2-dichloroethane.

[0002] The invention also relates to an apparatus for implementing themethod.

[0003] The products obtained from pyrolyzing 1,2-dichloroethane,hereafter denoted by DCE, contain in particular small quantities of1,3-butadiene, of the order of 10 to 100 parts per million (ppm),relative to vinyl chloride. These contents of 1,3-butadiene, denotedhereafter by BD, which are found in the final vinyl chloride aresufficient to inhibit its polymerization and it is necessary to reducethe BD contents to less than 8 ppm in order to make the vinyl chloridepolymerizable under industrial conditions.

[0004] It has often been found that the content of BD formed duringpyrolysis of DCE is lower the purer said DCE used, but it is alwaysnecessary to limit the BD content in the vinyl chloride obtained bypyrolysis of DCE in order for vinyl chloride polymerization to be ableto proceed normally.

[0005] To do this, many methods have been proposed. In the majority ofcases, these methods involve, in order to remove the BD, chlorination,hydrochlorination or hydrogenation reactions with or without thepresence of catalysts.

[0006] Thus, patent FR 1 216 030 proposes to purify the vinyl chlorideobtained by pyrolysis of DCE by making the vinyl chloride pass as amixture with hydrochloric acid, in the vapor phase, over ahydrochlorination catalyst such as HgCl₂ or FeCl₃. However, this methodrequires the inclusion of an additional catalytic treatment plant,thereby possibly considerably increasing the production costs, andrequires least some of the unconverted DCE to be separated from thepyrolysis products.

[0007] In U.S. Pat. No. 3,125,607, anhydrous chlorine is added to thevinyl chloride at a temperature between. −20°C. and 0° C. in a ratio ofchlorine to BD to be removed of about 5/1. The BD content passes from200 ppm to almost 0 ppm after 30 minutes of chlorination.

[0008] However, this method has the drawback of including an additionaldistillation for removing the chlorinated products from the BD, theexcess chlorine and various impurities liable to be formed during BDchlorination.

[0009] U.S. Pat. No. 3,125,608 discloses a method of purifying gaseousvinyl chloride containing BD, by heating said vinyl chloride withhydrogen to a temperature ranging from 60° C. to 250° C. in the presenceof a catalyst consisting of Pd supported by activated alumina.

[0010] This method has the drawback of including an additionalhydrogenation plant.

[0011] According to the method described in U.S. Pat. No. 3,142,709, thevinyl chloride is purified of BD by bringing liquid vinyl chloride intocontact with an amount of anhydrous hydrochloric acid representing from0.5 to 5% by weight of the vinyl chloride for a period ranging from 2minutes to 5 hours. However, this method applies only to the vinylchloride already separated from the other pyrolysis products andinvolves an additional distillation.

[0012] Patent FR 1 602 522 discloses a method for obtaining vinylchloride immediately capable of polymerization, which consists, startingfrom the raw product derived from the pyrolysis of DCE, in condensing,under a pressure greater than atmospheric pressure, said raw product inorder to obtain a liquid phase that is left to mature for at least twohours at a temperature between 0° C. and 100° C. before separating theconstituents thereof.

[0013] According to that method, the liquid phase after such a treatmentcontains practically no BD. In practice, this liquid phase remains intanks.

[0014] This method applies most particularly to pyrolysis productscontaining at most 10 ppm BD relative to vinyl chloride.

[0015] When the pyrolysis products contain larger amounts of BD, such asfor example at most 20 ppm, the process is carried out in the followingmanner.

[0016] After the prior condensation step, which is carried out bysuitably cooling the gaseous pyrolysis products maintained at a pressureabove atmospheric pressure, a gas phase is obtained which is inequilibrium with the liquid phase and contains a certain amount of BD.The matured liquid phase is used as liquid for scrubbing said gas phase,so as to absorb the residual BD. It then undergoes further maturing forat least two hours. Thus, by repeating this operation, in one or morecolumns, depending on the BD content in the raw pyrolysis product, it ispossible to completely purify vinyl chloride of BD.

[0017] Although giving satisfactory results, these various methods ofoperation have many drawbacks.

[0018] For low BD contents, this method applies only to the liquid phaseobtained after condensation of the raw product derived from thepyrolysis of DCE and does not take account of the BD contents of the gasphase that may fluctuate and consequently be found in the final vinylchloride.

[0019] When the amounts of BD are higher, at least one operation ofscrubbing the gas phase with the liquid phase matured for at least twohours is carried out, which liquid scrubbing phase is again matured forat least two hours.

[0020] This process involves many additional items of equipment and thelong residence times result in large volumes of inflammable and toxicproducts under pressure.

[0021] The Applicant has found that it is possible to obtain a vinylchloride capable of polymerization without the drawbacks of theaforementioned methods, by using simple and effective means for reducingthe 1,3-butadiene (BD) content of the entire stream coming from thepyrolysis of DCE.

[0022] The subject of the invention is therefore a method for obtainingvinyl chloride immediately capable of polymerization from all of the rawproduct derived from the pyrolysis of DCE having undergone a coolingoperation, said method being characterized in that all of the cooled rawproduct, consisting of a liquid phase and a gas phase, is cooled furtherdown to a temperature of at most 40° C. under a pressure ranging from 10to 15 bar, and then recovered from a tank at substantially identicaltemperatures and pressures, in which tank the liquid phase remains for atime of at most 20 minutes, and preferably for a time ranging from 5 to15 minutes, before separation of the constituents.

[0023] According to the present invention, the expression “vinylchloride capable of polymerization” denotes a vinyl chloride having a BDweight content of at most 8 ppm and preferably between 2 and 7 ppm.

[0024] According to the present invention, the gas stream coming fromthe pyrolysis furnace(s) undergoes suitable cooling, which consists of asuccession of quenching and condensation operations, which results in amixture in which the gas phase is in equilibrium with the liquid phase.This mixture—the cooled raw product—comprises vinyl chloride,hydrochloric acid, unconverted DCE, BD and various byproducts. Theamounts of BD may vary widely and depend in particular on the purity ofthe DCE used and on the conditions under which said DCE is pyrolyzed.They are generally between 10 ppm and 100 ppm in current methods.

[0025] According to the present invention, all of the cooled raw productis subjected to a further cooling operation under a pressure of greaterthan atmospheric pressure in a heat exchanger of the falling-filmexchanger type from which it leaves at a temperature of at most 40° C.The pressure within said exchanger is at most 15 bar, and preferablybetween 10 and 15 bar.

[0026] All of the cooled raw product thus treated is immediatelycollected in a tank under pressure and temperature conditions almostidentical to those under which it was cooled in the falling-filmexchanger, in which tank the liquid phase remains for a time of at most20 minutes and preferably for a time ranging from 5 to 15 minutes.

[0027] Next, the constituents of the mixture are separated using meansknown per se. According to the present invention, it is preferredhowever to separate the hydrochloric acid from the DCE/vinyl chloridemixture in a first distillation column, then the vinyl chloride from theDCE in a second column and, finally, the last traces of HCl are removedin a suitable apparatus.

[0028] The vinyl chloride coming from the third column advantageouslyhas a BD content of less than 8 ppm and is then immediately capable ofpolymerization.

[0029] The invention also relates to an apparatus for implementing themethod.

[0030] This apparatus (1), placed in a vinyl chloride production plantas shown schematically in FIG. 1 (the dotted area), inserted between acooling zone (7), for cooling the gas stream (GS) coming from thepyrolysis zone (8), and the HCl distillation column (9), consists of afalling-film heat exchanger (2) placed on top of a receiving pot (3),this being a horizontal cylindrical tank. The exchanger (2) is fed withthe entire cooled raw product (at 7) via a line (4). The HCldistillation column is fed with raw product treated in the apparatus (1)with, on the one hand, liquid product via the line (5) and, on the otherhand, gaseous product via the line (6). Withdrawn from the column (9) astops product is HCl and as bottoms product a mixture comprising vinylchloride, unconverted DCE and various byproducts, which mixture feedsthe vinyl chloride distillation column (10) via a line (12), from whichcolumn is withdrawn, as tops product, vinyl chloride (14) that ispurified in a suitable apparatus (11), from which apparatus it iswithdrawn at (16) and that is capable of industrial polymerization.

[0031] The unconverted DCE and various byproducts are removed at (13).

[0032] The residual HCl is withdrawn at (15).

[0033] The apparatus according to the invention is designed in such away that the exchanger is placed on the receiving pot in such a way thatthe products from the exchanger flow directly into said pot.

[0034] The falling-film exchanger consists of vertical tubes enclosed ina calender, through which tubes the liquid and gas streams flow, theliquid trickling down the walls of said tubes. Cooling is provided bycooling water flowing on the calender side. A person skilled in the artwill determine the necessary number of tubes suitable for the streams tobe treated and for the cooling.

[0035] The method and the apparatus for implementing the methodaccording to the invention allow the BD content in vinyl chloride to belowered, making it immediately polymerizable. They also have theadvantage of relieving the refrigeration demand at the top of the HCldistillation column. The example that follows illustrates the invention.

EXAMPLE

[0036] The example is carried out in an industrial plant as shownschematically in FIG. 1 and designed in such a way that it can produce50 metric tons of immediately polymerizable vinyl chloride per hour.

[0037] The pyrolysis zone (8), consisting of three furnaces, is fed withDCE having a purity of 99.7%.

[0038] This zone is maintained at a mean temperature of about 480° C.and at a pressure of 30 bar.

[0039] The stream coming from this pyrolysis zone, which comprises vinylchloride (VC), HCl, unconverted DCE (about 50%), BD and otherbyproducts, leaves at a temperature close to 480° C. and at a pressureof 30 bar and passes into a cooling zone (7), consisting of quenchingcolumns, which lowers its temperature to around 180° C. and into variousexchangers with various cold levels, which lower its temperature from180° C. to about 60° C.

[0040] All of the raw product thus cooled that leaves (7), consisting ofa gas phase comprising vinyl chloride, HCl, and small amounts of DCE,and of a liquid phase, comprising DCE, vinyl chloride and small amountsof HCl, is introduced into the top of the falling-film exchanger (2) ata temperature of about 56° C. This exchanger consists of 1652 tubes andis cooled with 28° C. water flowing countercurrently through thecalender. The pressure inside the falling-film exchanger (2) is 14 bar.

[0041] The stream leaves (2) at a temperature of 37° C. and a pressureof 14 bar and falls directly into the pot (3), which is a simplehorizontal cylindrical tank, in which the liquid phase remains for 15minutes at approximately 37° C. and a pressure of 14 bar. The gas andliquid streams are then directed to the distillation column (9) via thelines (6) and (5) respectively. In this column, HCl is separated at thetop at about 25° C. and a pressure of 13 bar.

[0042] The liquid phase, comprising VC, DCE and various byproducts iswithdrawn from the boiler maintained at about 110° C. and sent into thedistillation column (10) via the line (12).

[0043] In this column, the VC is separated at the top at 30° C. and apressure of 4 bar.

[0044] The bottom of the column (10) maintained at 150° C., essentiallycontains DCE and various byproducts that are withdrawn at (13). The VCis then sent via the line (14) into the column (11) where the residualHCl is removed, said HCl being withdrawn as top product (15). The VCwithdrawn from this column at (16) has a mean BD weight content rangingfrom 3 to 6 ppm and, as a consequence, is immediately polymerizable onan industrial scale.

1. A method for obtaining vinyl chloride for polymerization from all ofraw product derived from pyrolysis of 1,2-dichloroethane havingundergone a cooling operation, said method comprising cooled rawproduct, consisting of a liquid phase and a gas phase, cooled furtherdown to a temperature of at most 40° C. under a pressure ranging from 10to 15 bar, and then recovered from a tank at substantially identicaltemperatures and pressures, in which tank the liquid phase remains for atime of at most 20 minutes before separation of constituents.
 2. Themethod as claimed in claim 1, wherein the liquid phase remains in thetank for a time ranging from 5 to 15 minutes.
 3. The method as claimedin claim 1, wherein distillation under pressures greater thanatmospheric pressure.
 4. An apparatus for implementing the method asclaimed in wherein it consists of a falling-film exchanger placed on topof a receiving pot, these items of equipment being inserted between acooling zone, for cooling the gas stream coming from a pyrolysis zone,and a distillation zone comprising an HCl distillation column, a vinylchloride distillation column and a suitable device for eliminatingtraces of HCl.